deal with the case where model_expression is None
deal with the case where model_expression is None

apdjustino/urbansim,waddell/urbansim,bricegnichols/urbansim,SANDAG/urbansim,synthicity/urbansim,ual/urbansim,VladimirTyrin/urbansim,waddell/urbansim,synthicity/urbansim,apdjustino/urbansim,UDST/urbansim,apdjustino/urbansim,AZMAG/urbansim,UDST/urbansim,waddell/urbansim,bricegnichols/urbansim,VladimirTyrin/urbansim,ual/urbansim,VladimirTyrin/urbansim,AZMAG/urbansim,SANDAG/urbansim,UDST/urbansim,ual/urbansim,SANDAG/urbansim,AZMAG/urbansim,synthicity/urbansim,VladimirTyrin/urbansim,waddell/urbansim,bricegnichols/urbansim,AZMAG/urbansim,apdjustino/urbansim,UDST/urbansim,ual/urbansim,SANDAG/urbansim,synthicity/urbansim,bricegnichols/urbansim